In recent years, by focusing attention to the excellent characteristic of an oxide semiconductor made of indium gallium zinc oxide (hereinafter, referred to as “IGZO”) and the like, there has been progressed the development of a thin film transistor (hereinafter, referred to as “TFT”) that has the oxide semiconductor as a channel layer.
To increase the current driving force of a TFT, it is most effective to shorten the channel length. The channel length is equivalent to the length between the end part of the source electrode and the end part of the drain electrode, and this length is determined by a resolution limit of the exposure device. The resolution limit is determined by the wavelength of an exposed beam that is used in the exposure device. As the wavelength of the exposed beam is shorter, the resolution limit improves. Specifically, when the exposure device that can expose by using the g-line (a wavelength of 436 nm) and the h-line (a wavelength of 405 nm), (hereinafter, referred to as “g-line+h-line exposure device”), is used, the resolution limit is 3 μm. On the other hand, when the exposure device that can expose by using the i-line (a wavelength of 365 nm), (hereinafter, referred to as “i-line exposure device”), is used, the resolution limit improves to 1.5 μm. Accordingly, a channel length of the TFT taking the variation in the photolithography process into account can be shortened to about 2 μm when the i-line exposure device is used, while a channel length is about 4 μm when the g-line+h-line exposure device is used.
However, the i-line exposure device is more expensive than the g-line+h-line exposure device, and a large-type i-line exposure device that can expose a large-type substrate becomes more expensive. Therefore, to reduce as much as possible the manufacturing cost of forming a TFT on a large-type substrate of the G8 size (2200 mm×2500 mm), a large-type g-line+h-line exposure device is used in many cases instead of the large-type i-line exposure device. In this case, a channel length of the TFT becomes 3 μm to 4 μm (inclusive) as described above. Consequently, when using the g-line+h-line exposure device, a method for shortening the channel length is also required.
In Patent Document 1, to increase the current driving force of the TFT having a channel layer made of an oxide semiconductor, there is disclosed a technique of lowering the resistance of the oxide semiconductor layer by reducing the oxide semiconductor layer which is in contact with the source electrode and the drain electrode, thereby lowering the contact resistances between the source electrode and the channel layer and between the drain electrode and the channel layer.